Energy Value Problems

In the flat plate residential model, there seems to be an issue with the calculated energy value for net metered systems. It would be nice to be able to send this field to excel with equations so the calculation could be verified, but this seems to be unsupported. No matter how much I simplify the model (no tiers, escalation, inflation, load normalization, etc) to take out variables, it appears to be recognizing 87.3871% of what I have for the buy rate (and the system is producing less than is being bought monthly throughout the year) in the energy value field.

Hi Jay,

Would you mind attaching a copy of your .zsam file to your original message? If you would prefer not to attach it here, please email it to me at sam dot support at nrel dot gov.

Best regards,
Paul.

I attached this file to the original message. Please take a look at the energy value which I would expect to be $.10/kWh listed right above, but is reduced for some reason.

Thanks,

Dear Jay,

Thank you for attaching your file.

First of all, if you are still using SAM 2013.9.20, please be sure to install the current version, SAM 2014.1.14, which is a maintenance update that fixes a couple of bugs with the residential/commercial financial models, including one that affects the net metering calculations.

For the residential and commercial financial models, the "energy value" is the net savings, or difference between the electricity cost without the system and the electricity cost with the system. In other words, the energy value is the value of the electricity that the household or business avoids purchasing because of the solar system. The electricity cost without the system is what it would cost to purchase electricity to meet 100% of the load at the rates specified on the Utility Rate page. The cost with the system is what it costs to purchase electricity to meet the portion of the load that the solar system cannot meet.

In your file, the annual cost of purchasing electricity without the system is $1,130.80. That makes sense because the annual load is 11,308 kWh and the electricity rate is $0.10/kWh.

If you turn off net metering in your file, and set the sell rate for Period 1 Tier 1 and Period 2 Tier 1 to $0.10 so that the buy and sell rates for those periods are equal, you pay $0.10/kWh for the electricity you purchase from the utility, and you pay $0.10/kWh for any electricity you sell to the utility. With the system, you reduce the total annual load by the system's output: 11,308 kWh - 7,144 kWh = 4,164 kWh. The net savings is $1,130.80 - ( $0.10/kWh * 4,164 kWh) = $714. In other words, with no net metering and the same buy and sell rate, the system effectively sells all of the system's output at the sell rate.

When you enable net metering (see "Example Residence - Net Metering Enabled" in the SAM file), the system still generates 7,144 kWh per year, but the energy value is $653.47 because the net metering credit is based on a monthly accounting of surplus generation.

Please see the Excel file and modified version of your SAM file attached to your original message.

Best regards,
Paul.

Thank you. I think the original source of my confusion is that you don't appear to be including the net metering year end sale of excess energy in the (yearly) energy value displayed in the cash flow chart. If the year end sell rate was the same as the buy rate under net metering, I would have expected the Energy Value to be unchanged.

That leads me to wonder if the year end sale of energy is included in the NPV and payback calculations at all as it appears it is not.

I also mistakenly sent you an example where there was excess production in 2 of 12 month which I was trying to avoid.

Thanks

Hi Jay,

The year-end sell rate should affect the energy value number, but only if the system generates excess electricity at the end of the year. For example, in the "Residential Net Metering and Tiered TOU Example" sample file, "Net Metering with Excess Generation" case, the Year End Sell Rate input is $0.05/kWh, which results in a net savings of $897.59. If I change the Year End Sell Rate value to zero, the net savings changes to $887.20. In that example, the year-end net metering reconciliation is worth about $10.

In your file, the load is greater than the system's output in November and December, so there are no net metering credits to reconcile at the end of the year.

Best regards,
Paul.